In 1995, Williams and Yates proposed an electromechanical generator to convert vibrational energy to electrical energy. C. B. Williams and R. B. Yates, “Analysis of a micro-electric generator for Microsystems,” Proc. Transducers '95/Eurosensors IX, pp. 369–372 (1995) (incorporated herein by reference). As shown in FIG. 1, this generator consisted of a spring-mounted mass having a wire coil attached thereto. The free end of the spring was attached to a housing. Thus, when the housing vibrated the mass would oscillate so that the coil of wire would move through a magnetic field created by a permanent magnet positioned at the opposite end of the housing from which the spring-mounted mass was suspended. This caused an electrical current to be produced in the coil and a voltage was thereby available the output of the generator. Indeed, according to Williams and Yates, a device that measured 4 mm×4 mm×1 mm produced 0.3 μW of power (a power density of approximately 100 μW/cm3).
In 1998, Amirtharajah and Chandrakasan showed that it would be possible to use the electromechanical generator of Williams and Yates to power a low-power digital electronic circuit. Rajeevan Amirtharajah and Anantha P. Chandrakasan, “Self-powered signal processing using vibration-based power generation,” IEEE J. Solid-State Circuits, vol. 33, no. 5, pp. 687–695 (May 1998) (incorporated herein by refernce). The entire power supply system included no only the electromechanical generator, but also a voltage rectifier and regulator system. Generated power on the order of 400 μW (with a maximum output voltage of approximately 180 mV) was found to be feasible for a device measuring 4 cm×4 cm×10 cm.
Other methods to produce electrical energy from vibrational energy have also been discussed. For example, Meniger et al. described the use of a microelectromechanical systems technology (MEMS) variable capacitor to convert ambient mechanical vibration into electrical energy. Scott Meniger, et al., “Vibration-to-electric energy conversion,” IEEE Trans. VLSI Systems, vol. 9, no. 1 pp. 64–76 (February 2001) (incorporated herein by reference). In this scheme, the vibrational energy was transduced through a MEMS capacitor etched on a silicon wafer. This device included a floating mass (free to move in one dimension), a folded spring (one per side) and two sets of interdigitated combs (one per side). As the mass oscillates, the interdigitated combs move together and apart, effectively varying the capacitance of the variable capacitor. This change in capacitance results in power being made available to an electronic circuit attached to the variable capacitor. In fact, Meninger et al. reported that up to 8.6 μW of power (3.8 μW/cm3) would be expected from such a device that measured only 1.5 cm×1.5 cm.
Piezoelectric materials also offer opportunities to convert mechanical energy to electrical energy. For example, Kasyap et al. described an energy reclamation device that allowed electrical energy to be obtained from piezoceramic composite cantilever beams coupled to shunt circuits. Anurag Kasyap et al., “Energy reclamation from a vibrating piezoceramic composite beam” (incorporated herein by reference). This generator used an AC-to-DC flyback converter to provide an impedance match between the piezoelectric transducer and the output load, and to supply a DC voltage thereto.
Thus, it has been generally recognized that mechanical energy in the form of vibrational energy can be used as a power source for producing electrical energy. Aside from not-yet-realized, human-wearable computer systems, however, there seems to have been little or no discussion of applications for such technologies. This may be due, in part, to the relatively small amounts of power that have been generated using such systems.